122 A TEXTBOOK OF OCEANOGRAPHY 



to north. A detailed consideration of these derived waves is 

 impossible here,* but a brief account of the tides and tidal 

 streams of the British Isles is given below. 



The Tides of the British Isles. 



The main tidal undulation from the Atlantic approaches 

 the British Islands from a south-westerly direction, the line of 

 its Crest running north-west and south-east. 



It makes high-water on the Atlantic coasts of the British 

 Isles, on full and change days, at from 4^ o'clock to 6J o'clock 

 Greenwich mean time ; it reaches Ushant a little before 4, the 

 south-west of Ireland at 4.30, Scilly Islands at 4.50, the north- 

 western extremity of Ireland at 5.30, and the outer Hebrides 

 at 6.30, Greenwich mean time (Fig. 21). 



The undulation is practically unobstructed in ocean depths, 

 and here it does not appear to exceed 5 feet in height, and is 

 probably much less. 



When it reaches the continental shelf on which the British 

 Isles are situated it becomes obstructed and its progress is 

 retarded. Ultimately it divides into three main streams, one 

 up the English Channel, one up the Irish Channel, and the 

 third up the trough to the west of Ireland lying between the 

 Scottish coast and Rockall, and extending northwards towards 

 the Faroes. 



The tidal undulation has a greater elevation at its south- 

 east part than elsewhere, the height of the crest above the 

 trough being about 10 feet at the south-west of Ireland and 

 19 feet at Ushant ; and this peculiarity is preserved in its 

 progress up the English, Bristol, and Irish Channels, the 

 range of the tide being always greater on the right or south- 

 eastern part of the advancing wave. This is reversed when 

 the wave has rounded the north coast of Scotland ; then the 

 western part of the undulation is higher than the eastern. 



There are three greater and three lesser maxima developed 

 viz. : 



* See Harris, " Manual of the Tides." 



